1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device using a semiconductor thin film, and particularly to a method of manufacturing a thin film transistor (TFT) using a crystalline film containing silicon.
Incidentally, the term xe2x80x9csemiconductor devicexe2x80x9d used in the present specification includes all devices functioning by using a semiconductor, and not only a single element such as a TFT, but also an electro-optical device and an electronic device equipped therewith are also included in the category of the semiconductor device.
2. Description of the Related Art
In recent years, a technique of constituting a semiconductor circuit by forming TFTs on a glass substrate and the like, has been rapidly developed. As such a semiconductor circuit, an electro-optical device such as an active matrix type liquid crystal display device is typical.
The active matrix type liquid crystal display device is a monolithic display device in which a pixel matrix circuit and a driver circuit are provided on the same substrate. Moreover, a system-on-panel with additional built-in logic circuits of a memory circuit, a clock generating circuit and the like has also been developed.
Since such a driver circuit and a logic circuit are required to be operated at high speed, it is not suitable to use a noncrystalline silicon film (amorphous silicon film) as an active layer. Thus, under the present circumstances, a TFT using a crystalline silicon film (polysilicon film) as an active layer has become the main stream.
In general, the crystalline silicon film is obtained by forming an amorphous silicon film on a glass substrate or a quartz substrate and crystallizing the amorphous film by irradiation of a laser beam or heating.
Since the substrate is hardly heated when the crystalline silicon film is obtained by the irradiation of a laser beam, the glass substrate can be used as the substrate. However, the crystallinity of the obtained crystalline silicon film is not so good. The characteristics of a TFT obtained by using the crystalline silicon film also becomes unsatisfactory.
On the other hand, the method of heating has a problem that necessary crystallinity can not be obtained by a heat treatment at such a temperature that the glass substrate can withstand.
There is also a method in which a quartz substrate is used and a crystalline silicon film is obtained by a heat treatment at such a high temperature as 900xc2x0 C. or more (a silicon film obtained by this method is especially called a high temperature polysilicon).
However, according to this method, the precipitation of grain boundaries is remarkable, and by this influence, the electrical characteristics of the obtained semiconductor device is not satisfactory.
The present inventors disclose a technique for obtaining a crystalline silicon film on the glass substrate in Japanese Patent Unexamined Publication Nos. 7-321337 and 8-78329, the disclosure thereof being incorporated herein by reference. In the technique disclosed in the publications, a catalytic element for promoting crystallization is selectively added into an amorphous silicon film, and by carrying out a heat treatment, a crystalline silicon film extending from the starting point of the added region is formed.
This technique can lower the crystallization temperature of the amorphous silicon film by the action of the catalytic element drastically by 50 to 100xc2x0 C., and the time required for crystallization can also be reduced to ⅕ to {fraction (1/10)}. Since the crystallization of the silicon film progresses in a lateral direction substantially parallel to the surface of the substrate, the present inventors call this crystallized region a lateral growth region.
Since the catalytic element is not directly added in the lateral growth region, the lateral region has a feature that the amount of the catalytic element remaining in the film is small compared with the case directly added. For example, although a region in which the catalytic element is directly added, includes the catalytic element in the order of 1019, the lateral growth region includes the catalytic element in the order of 1018 which is smaller than the former value by one figure.
As the above-mentioned catalytic element, a metal element such as nickel, cobalt and tin is used. Since such a metal element forms a deep level in a silicon film to capture a carrier, there is a fear that the metal element has a bad influence to the electrical characteristics and reliability of a TFT. This problem is not exceptional even for the above-mentioned lateral growth region.
An object of the present invention is to provide a technique to remove the influence of a metal element when a semiconductor device is manufactured by using a crystalline silicon film obtained by using the metal element.
In order to achieve the object, according to a first aspect of the present invention, a method of manufacturing a semiconductor device comprises the steps of: introducing a metal element for promoting crystallization of silicon into a partial region of an amorphous silicon film; causing crystal growth from the partial region in a direction substantially parallel to a surface of the amorphous silicon film by a heat treatment to obtain a silicon film in which at least a partial region is crystallized; doping a part of the silicon film with phosphorus; and moving the metal element by a heat treatment into a region which has been doped with the phosphorus, wherein a thin film transistor having a channel made of the region where the crystal growth has been carried out and the metal element has moved into the region which has been doped with the phosphorus, is manufactured, and wherein in the region of the channel, an axis coincident with a direction of crystal growth is substantially made to coincide with an axis coincident with a direction of movement of the metal element into the region doped with the phosphorus.
In the above structure, as a method of selectively introducing the metal element, there is enumerated a method in which a nickel element is selectively brought into contact with the surface of the amorphous silicon film and is held by a plasma treatment, a CVD method, a sputtering method or a method using a solution after providing a mask. There is also enumerated a method of selectively implanting an ion of the metal element using a mask.
The step of causing crystal growth from the partial region in the direction substantially parallel to the surface of the film by the heat treatment to obtain the silicon film in which at least the partial region is crystallized, means that as shown in FIG. 3 in which the state of crystal growth is schematically shown, the crystal growth is made in a direction 301 parallel to the surface of the silicon film from a region 104 where the metal element has been introduced.
In this case, crystal growth is not necessarily made in all surfaces of the film.
As the doping method of phosphorus, although a plasma doping method and an ion implantation method in which a phosphorus ion is accelerated and implanted, are general, a diffusion method or the like may be used.
The step of moving the metal element into the region doped with phosphorus means that as schematically shown in FIG. 3B, the metal element, which was once diffused at the crystal growth (the direction of the diffusion is substantially coincident with the direction of crystal growth as indicated by 301), is made to move by a heat treatment into regions 108 and 109 doped with phosphorus.
Especially, the feature of the present invention mentioned above is that in the region which becomes the channel, an axial direction (one-dimensional extending direction) coincident with the direction 301 of the crystal growth in FIG. 3 is substantially made to coincide with an axial direction (one-dimensional extending direction) coincident with the direction 302, 303 of movement of the metal element carried out after crystallization.
In the case where the above-mentioned two axial directions are coincident with the moving direction of a carrier, the possibility of being scattered and trapped at the movement of the carrier can be remarkably lessened, so that a device having high electrical characteristics can be obtained. This is caused by the fact that when the metal element moves, the crystallinity in the moving direction is dramatically improved in the path of the movement.
According to another aspect of the present invention, a method of manufacturing a semiconductor device using a silicon film obtained by using a metal element for promoting crystallization of silicon, comprises the steps of: causing crystal growth by moving a selectively introduced metal element; and removing the metal element by moving the metal element; wherein a moving direction of a carrier in a channel is set in a direction in which an axis coincident with a direction of movement of the metal element at crystallization is substantially coincident with an axis coincident with a direction of movement of the metal element at the removable of the metal element.
In the present invention, it is extremely preferable to use nickel as the metal element.
One kind or plural kinds of metal element selected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au may be used as the metal element for promoting crystallization of silicon.
Other elements that have a gettering effect may be used. For example, nitrogen, arsenic, antimony, and bismuth may be used.
According to a still another aspect of the present invention, a method of manufacturing a semiconductor device comprises the steps of: selectively forming an insulating film on an amorphous film comprising silicon; selectively holding or adding a catalytic element for promoting crystallization of silicon to the amorphous film by using the insulating film as a mask; transforming at least a part of the amorphous film into a crystalline film by carrying out a heat treatment; selectively holding or adding an element selected from group 15 elements of the Periodic Table in the crystalline film by using the insulating film as a mask as it is; and carrying out a heat treatment so that a region where the element selected from the group 15 has been held or added, is made to getter the catalytic element.
According to a still another aspect of the present invention, a method of manufacturing a semiconductor device comprises the steps of: selectively forming an insulating film on an amorphous film comprising silicon; selectively holding or adding a catalytic element for promoting crystallization of silicon to the amorphous film by using the insulating film as a mask; transforming at least a part of the amorphous film into a crystalline film by a heat treatment; selectively holding or adding an element selected from the group 15 elements in the crystalline film by using the insulating film as a mask as it is; and carrying out a heat treatment so that a region where the element selected from the group 15 elements has been held or added, is made to getter the catalytic element, wherein the region where the catalytic element is held or added is the same as the region where the element selected from the group 15 is held or added.
The basic object of the present invention is to remove the catalytic element, which was used for crystallization of the amorphous film comprising silicon, from the crystalline film, and the gettering effect of the element selected from group 15 elements is used as the means for removing the catalytic element.
As the catalytic element, Ni (nickel), Co (cobalt), Fe (iron), Pd (palladium), Pt (platinum), Cu (copper), and Au (gold) are typical. However, other metals or semimetals may be used as the catalyst.
As the element in group 15 elements for gettering the catalytic element, N (nitrogen), P (phosphorus), As (arsenic), Sb (antimony), and Bi (bismuth) are enumerated. Especially, phosphorus has a remarkable function and effect.
As a typical example, when nickel is used as the catalytic element, and phosphorus is used as the gettering element, by a heat treatment at about 600xc2x0 C., phosphorus and nickel show a stable coupling state. At this time, a coupling state such as Ni3P, Ni5P2, Ni3P2, Ni2P3, NiP2, NiP3 can be formed.
As described above, when nickel is used as the catalytic element for promoting crystallization of an amorphous film comprising silicon, by the action of phosphorus, nickel can be gettered. By using this effect, it is possible to remove the catalytic element or lower its concentration in the crystalline film.
The most remarkable features in the above-mentioned invention are that:
1. a crystalline film called a lateral growth region is formed by selectively providing an insulating film (mask) to an amorphous film and by holding or adding a catalytic element, and
2. a gettering element is held or added by positively using the mask as it is, to getter the catalytic element remaining in the lateral growth region.
That is, when the amorphous film is crystallized, by using the technique disclosed in Japanese Patent Unexamined Publication No. 8-78329, the insulating film remaining on the crystalline film after crystallization is again used as a mask for selectively holding or adding the gettering element. The disclosure of said publication is incorporated herein by reference. Thus, the region where the catalytic element is held or added is the same as the region where the gettering is held or added.